conventionalmillimeters of mercury

Convert
between millimeters of mercury and other major units of pressure.

A unit of pressure, obsolete since 1960, about 133.322
pascals. Symbol, mmHg. (Notice that there is no space between “mm” and
“Hg”). One mmHg is the pressure exerted at the bottom of a vertical column
exactly 1 millimeter deep of a fluid whose density is exactly 13.5951 grams per
cubic centimeter, at a location where the acceleration due to gravity is exactly
980.665 centimeters per second per second. The second exact value is, of course,
a value for the density of mercury at 0°C, and the third the conventional value
for the acceleration due to gravity at the Earth's surface. 1 mmHg = 13.5951 ×
9.806 65 newtons per square meter.

The mmHg is often described as synonymous with the torr,
but it differs from it, although “by less than 2 × 10−7 Torr.”1

According to the current national standard in the United States2,
the millimeter of mercury is not to be used. The pascal
should be used instead.

In Europe, Council Directive 80/181/EEC of 20 December 1979 allowed the
continued use of the millimetre of mercury (thus derogating Directive
71/354/EEC). Revisions of the directive continue to sanction the unit's
use, but that use is restricted to measurement of “blood pressure and pressure
of other body fluids.”3

1. Pure and Applied Chemistry, volume 21 (1970).

Page 24, footnote 1.

2. IEEE/ASTM SI 10™-2002.American National Standard for Use of the International System of Units (SI):
The Modern Metric System.
New York: IEEE, 30 December 2002.

See Section 3.3.3.

3. Council Directive 80/181/EEC as amended by February 2000. See
Annex, Chapter 1, Section 4, Units and Names of Units Permitted in Specialized
Fields Only

Blood pressure is conventionally measured in millimeters of mercury,
because for many years doctors measured blood pressure with a mercury
sphygmomanometer. This instrument contained a tube, one end of which was
submerged in a pool of mercury in an airtight chamber. The cuff was connected to
this chamber, and air pressure in the cuff also pushed on the pool of mercury,
forcing the mercury up the tube. A scale marked in millimeters placed next to
the column readily indicated the pressure in millimeters of mercury.